In this paper, we introduce environmental systems engineering (ESE) and describe some of its
applications combined with process systems engineering (PSE) to problems related to environmental systems. These systems—the water cycle and ecosystem cycles—are complex and highly dynamic, with an uncertainty level comparable to that of chemical systems. To illustrate the challenges of applying PSE to ESE, some novel approaches and examples of the latter are shown for water and wastewater systems. The challenges associated with the modeling, control and optimization of environmental systems provide fascinating opportunities. These opportunities for PSE researchers, as well as the challenges, are the goals of this paper.